Methymalonyl CoA mutase catalyzes the isomerization of methylmalonyl CoA to succinyl CoA which is an intermediate step in the degradation of branch chain amino acids, odd chain fatty acids, and cholesterol. Genetic deficiency of this enzyme causes methylmalonic acidemia, a disorder of organic acid metabolism associated with mental retardation, developmental delay, fulminant acidosis, or even neonatal death. Approximately 1:20-50,00 newborn suffer from this often fatal disorder. Methylmalonyl CoA mutase s one of two cobalamin (vitamin B12) requiring enzymes in the body. Thus, deficiency of B12 in pernicious anemia, or genetic deficiencies in B12 metabolism, also cause deficits in methylmalonyl CoA mutase activity. We have recently obtained a full length clone for methylmalonyl CoA mutase from human liver and demonstrated that this clone can be used to transduce MCM activity into cultured cells by genetic transfer. The availability of this clone enables molecular genetic studies of the structure, function, and evolution of methylmalonyl CoA mutase and the genetics, pathochemistry, diagnosis, and therapy of methylmalonic acidemia. Specifically, we propose to characterize the structure, of the methylmalonyl CoA mutase gene by mapping and sequencing both cDNA and genomic clones. We will use these clones as probes to identify mutations in patients with methylmalonic acidemia. We will produce recombinant methylmalonyl CoA mutase in vitro, in eukaryotic cell culture, or in prokaryotes in order to study the structure, processing, enzymatic activity and metabolic role of this enzyme. We will explore genetic means for suppression of methylmalonyl CoA mutase activity in cultured cells in order to create cellular models of methylmalonyl CoA mutase deficiency and study the secondary metabolic aberrations which contribute to the pathology of methylmalonic acidemia. We will develop recombinant viral vectors carrying methylmalonyl CoA mutase genes capable of reconstitution of enzyme activity in methylmalonyl CoA mutase deficient cells and explore the potential for somatic gene replacement therapy of methylmalonic acidemia.